3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB. A UE may more generally be referred to as a wireless device or a wireless terminal.
FIG. 1 illustrates a part of an LTE system. In the radio access network an eNodeB 101a serves a UE 103 located within the eNodeB's geographical area of service or the cell 105a. The eNodeB 101a is connected via an X2 interface to a neighboring eNodeB 101b serving another cell 105b. The two eNodeBs 101a and 101b are connected to a core network node called Mobility Management Entity (MME). The core network in LTE is sometimes referred to as Evolved Packet Core (EPC), and the MME is one of the core network nodes in EPC. Together, the E-UTRAN, the EPC and potentially other entities too, such as service related entities, are referred to as the Evolved Packet System (EPS). S1 Application Protocol (AP) provides the signaling service between E-UTRAN and the EPC. The Non-Access Stratum (NAS) protocol is used for the control signaling between the UE and the MME.
In a current vision of the future development of the communication in cellular networks, huge numbers of mostly small autonomous wireless devices become increasingly important. These devices are assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers within or outside the cellular network. The application servers configure the devices and receive data from them. Hence, this type of communication is often referred to as machine-to-machine (M2M) communication and the devices may be denoted machine devices (MDs). In the 3GPP standardization the corresponding alternative terms are machine type communication (MTC) and MTC devices. The MTC devices are a subset of the more general term UE. In terms of numbers, MTC devices will dominate over human users, but since many of them will communicate very scarcely, their part of the traffic volume will probably be small compared their part of the device population.
With the nature of MTC devices and their assumed typical usage follow that they will often have to be very energy efficient, since external power supplies will often not be available and since it is neither practically nor economically feasible to frequently replace or recharge their batteries. In some scenarios the MTC devices may not even be battery powered, but may instead rely on energy harvesting, i.e. gathering energy from the environment, opportunistically utilizing the often very limited energy that may be tapped from e.g. sun light, temperature gradients, and vibrations.
For such energy deprived devices the traffic is characterized by small, and more or less infrequent transactions that often are delay tolerant. As the transactions or communication events are infrequent, each of them will involve a process of establishing a connection to the network or—in case a connection can be maintained across multiple communication events—at least a Random Access (RA) procedure to synchronize the uplink, i.e. to acquire a valid timing advance. Hence, the behavior of these devices will result in a large, inevitably energy consuming signaling overhead.
Machine devices, however, consist of a very heterogeneous flora of devices and applications. Although the above described energy deprived devices, according to the vision, may constitute the largest part in terms of numbers, many other types of MTC devices and MTC applications are also envisioned and to some extent already existing. One example that has received quite a lot of attention is the development of power grids into so called “smart grids”. This refers to the evolution of the conservative power grid technology into grids that are better adapted to the envisioned future requirements in the area of generation and distribution of electricity. The future requirements involve intermittent generation sources such as wind and solar power plants, many small generation sources such as from customers which sometimes produce more electricity than they consume, and a desire to impact the customers energy consumption habits to even out load peaks. In this evolution, information technology and in particular communication technology has an important role to play. In many smart grid applications, entities in the power grid, so-called substations (e.g. transformer stations) communicate with each other and with a control center for the purpose of automation and protection of equipment when faults occur. In contrast to the above described energy deprived devices with delay tolerant scarce communication, these smart grid applications often have extremely strict latency requirements, the amount of data communicated may range between small and large and the energy supply is typically not an issue. To make cellular communication technology a possible and attractive means of communication for such devices and applications, it is crucial to keep the delay associated with access and end-to-end communication as low as possible.
Although smart grid applications are prime examples of a MD application scenario that requires low access delay, they are not the only ones. MDs are also likely to find many applications in industrial processes, where they may serve purposes such as tuning the process and/or detecting and/or initiating automatic actions upon fault conditions. Also in the latter example, there may be many scenarios where extremely swift reactions are needed in order to avoid that expensive equipment is damaged or that costly restarts of complex processes are needed.
A property that such low delay applications share with the above described energy deprived MTC devices is that they require that the signaling overhead involved in network access is minimized. The current network access signaling procedure, focusing on the RA part, is described hereinafter.
If the concerned device is in RRC_IDLE state, the network access signaling involves a transition to RRC_CONNECTED state. In this case the network access control signaling consists of a RA phase for synchronizing with and gaining initial access to the cellular network and a further phase for authentication, configuring the connection, and establishing appropriate states on higher layers. The connection may e.g. be a Radio Resource Control (RRC) connection, and the establishing of appropriate states on higher layers may e.g. be done through S1AP signaling between the eNodeB and the MME, and NAS signaling between the UE and the MME in EPS. A first part of the network access signaling procedure, involving the RA procedure, is illustrated in the signaling diagram in FIG. 2.
The RA procedure consists of the first four messages S1-S4 of the procedure illustrated in FIG. 2. The RA messages pertain to the Medium Access Control (MAC) layer. In the case of transition from RRC_IDLE to RRC_CONNECTED state, as in FIG. 2, S3—RA Msg3 and S4—RA Msg4 also carry RRC layer messages between the UE and the eNodeB. The most elaborate of the RA messages is S2—RA Msg2, denoted RA Response (RAR). The format of the MAC Packet Data Unit (PDU) for a RAR (RAR PDU) is illustrated in FIG. 3 and FIG. 4.
In FIG. 3, BI is the optional Backoff Indicator, RAPID is the RA Preamble Identity (ID) indicating the RA preamble transmission that the corresponding MAC RAR pertains to, E is the Extension flag indicating whether there are more subheaders in the MAC header, T is the Type flag indicating whether the subheader comprises a BI or a RAPID, the two R fields in the optional BI subheader are reserved bits set to zero by the sending eNodeB and ignored by the receiving UE, and MAC RAR comprises the actual response information to the UE.
FIG. 4 illustrates the format of a MAC RAR, according to 3GPP TS 36.321, V11.1.0, section 6.1.5. The MAC RAR consists of four fields: a reserved bit R, a Timing Advance Command, an Uplink (UL) Grant, and a temporary Cell Radio Network Temporary Identifier (C-RNTI).
As can be seen from FIG. 2, the RA procedure in LTE consists of:                S1. RA Message 1 (Msg1): The UE 103 transmits a preamble on the Physical RA Channel (PRACH) to the eNodeB 101. Each cell has its own set of 64 RA preambles. However, preambles may be reused between non-neighbor cells. The preambles may optionally be divided into two groups, A and B. The UE then selects a group to randomly pick a preamble from, based on the potential message size and the channel quality. The potential message size is the size of the data available for transmission in message three of the RA procedure (S3. RA Message 3) plus the size of the MAC header and any possible MAC control elements. The channel quality is estimated in terms of the measured downlink (DL) path loss. Two conditions have to be met for the UE to select a preamble from preamble group B: the potential message size has to exceed a certain threshold and the estimated path loss has to be lower than a certain threshold.        S2. RA Message 2 (Msg2): The eNodeB 101 sends a RA Response (RAR) to the UE 103 using a broadcast identifier, RA Radio Network Temporary Identifier (RA-RNTI). As indicated in FIG. 3 the RAR PDU may contain a BI and zero or more MAC RAR. Each MAC RAR comprises a temporary C-RNTI (TC-RNTI), a timing advance command, an UL grant and a reserved bit, R, which is set to zero. The MAC PDU header comprises one MAC subheader, i.e. one RAPID subheader, for each MAC RAR that is included in the RAR PDU. Each such corresponding RAPID subheader includes a RA preamble ID which indicates the received RA preamble that the corresponding MAC RAR pertains to. Hence, in this way each MAC RAR is mapped to a preamble transmitted by the UE and received by the eNodeB in step S1, and to a PRACH (see FIG. 3 and FIG. 4).        S3. RA Message 3 (Msg3): The UE 103 transmits RA Msg3 to the eNodeB 101 using the UL transmission resources allocated by the UL grant in step S2. The message comprises the RRC layer message RRCConnectionRequest, which includes a UE identity which may be an S-Temporary Mobile Subscriber Identity (S-TMSI) or a random value. The UE identity is used for contention resolution, i.e. to resolve situations where two or more UEs simultaneously used the same preamble in step S1. Depending on the parameters in the UL grant received in step S2, this Msg3 is transmitted 6 or 7 subframes after the reception of the RAR (RA Msg2) in a Frequency Division Duplex (FDD) mode. In Time Division Duplex (TDD) mode the timing also depends on the configuration of UL and DL subframes.        S4. RA Message 4 (Msg4): To conclude the contention resolution, the eNodeB 101 echoes the UE identity received in RA Msg3 in a UE Contention Resolution Identity MAC Control Element in RA Msg4. RA Msg4 also carries the RRC layer message RRCConnectionSetup as a part of the RRC connection establishment. The UE “promotes” the TC-RNTI received in RA Msg2 to a regular C-RNTI.        
The fifth message S5 is the RRCConnectionSetupComplete message from the UE 103 to the eNodeB 101. This message is not a part of the actual RA procedure although it is a part of the RRC connection establishment procedure. The RRCConnectionSetupComplete message also comprises the initial NAS message from the UE, forwarded to the MME in step S6.
The last message S6 illustrated in FIG. 2 is the S1AP Initial UE Message forwarded by the eNodeB 101 to the MME 102. This NAS message may be e.g. a Service Request as illustrated in FIG. 2. However, it may also be an Attach Request, or a Tracking Area Update Request message.
One common case where the RA procedure is used in conjunction with idle to connected mode transition is when the UE responds to a page from the network. To be reachable for paging, an idle UE has to monitor a certain repetitive DL signaling channel to check for paging indications directed towards it. In EPS this consists of monitoring the Physical DL Control Channel (PDCCH) for DL resource assignments—also called DL scheduling assignments—addressed to a Paging RNTI (P-RNTI). The P-RNTI is shared by many UEs, so when detecting such a paging indication, the UE has to receive the Paging RRC message, which is transmitted on the DL transmission resources on the Physical DL Shared Channel (PDSCH) that were assigned by the paging indication on the PDCCH. This Paging RRC message comprises the identity/identities of the UE(s) that the paging concerns and which is/are thus requested to contact the network. When finding its identity in a Paging RRC message, the UE initiates a RA procedure towards the eNodeB, establishes an RRC connection with the eNodeB and sends a Service Request NAS message to the MME.
Network access signaling comprising the RA procedure is involved also in the case where a UE that wants to transmit UL data is in RRC_CONNECTED state, but lacks UL synchronization, i.e. lacks a valid Timing Advance. A RA procedure will also be required even if a UE in RRC_CONNECTED state is synchronized with the UL, but has not been allocated any resources on the Physical UL Control Channel (PUCCH) for transmission of Scheduling Requests. The allocation of PUCCH resources is optional. This access signaling procedure is much briefer than during idle to connected mode transition. FIG. 5 illustrates the message sequence of the network access signaling involved when a UE in RRC_CONNECTED state that lacks a valid timing advance wants to transmit UL data in the FDD mode of EPS/LTE. Signaling in step S1 and S2 is the same as described above with reference to FIG. 2 during idle to connected mode transition. Whether the delay between step S2 and S7 is 6 or 7 subframes depends on the parameters of the UL grant in step S2. As previously mentioned, in TDD mode this delay also depends on the configuration of UL and DL subframes.
In step S7 of the message sequence of FIG. 5 the UE transmits UL data on the PUSCH utilizing the transmission resources allocated in step S2. This is opposed to the case of idle to connected mode transition where the UE transmits an RRCConnectionRequest message in step S3. Since the UE is in RRC_CONNECTED state, it already has a valid C-RNTI. The UE identifies itself in step S7 by including its C-RNTI in a C-RNTI MAC Control Element. If there is more room available in the allocated transmission resources, the UE may also include a Buffer Status Report (BSR) and/or user data according to the following rule: If all user data can be fit in the transport block, then inclusion of user data has priority over inclusion of a BSR, but if not all user data can be fit in the transport block, then the BSR has priority over the user data.
Step S8 is the contention resolution step, comprising an UL grant from the eNodeB 101 addressed to the C-RNTI transmitted from the UE in step S7.
If two or more UEs select and transmit the same preamble in the same subframe, they will listen to the same response message in step S2 and will thus attempt to transmit in step S7 using the same UL resources. This results in a RA collision and a contention situation. Such collisions cause transmission failures and in the worst case the eNodeB will not be able to successfully receive and decode any of the UEs' transmissions. A UE for which the contention resolution implies RA failure will reattempt the RA procedure, starting at step S1, possibly after a random backoff delay if a BI was included in the RA Response message in step S2.
A third variant of the RA procedure is used in conjunction with handover from one cell to another, e.g. between a source cell and a target cell belonging to two different eNodeBs. In this case the UE is allocated a dedicated, contention-free RA preamble prior to the execution of the handover. This is done in a mobilityControlInfo Information Element (IE) in an RRCConnectionReconfiguration message in the source cell. The dedicated, contention-free RA preamble is allocated by the target eNodeB and transferred to the source eNodeB before it is conveyed to the UE. The purpose of the subsequent RA procedure in the target cell is to acquire a timing advance in the new cell. Since the UE uses a dedicated, contention-free preamble, the risk for RA collisions is eliminated and the RA procedure may therefore be limited to the first two steps S1 and S2, i.e. RA Msg1 and RA Msg2.
As mentioned previously, it is of crucial importance for many MD applications that the network access control signaling is minimized. The problems caused by a large control signaling overhead include both energy consumption and access delay.